A primary goal of cosmetic science is improvement of the outward appearance and health of skin. While the visible signs of aging cannot be fully avoided, they can be dramatically minimized. Less desirable skin traits include wrinkles, fine lines, age spots, uneven skin tone, and loss of firmness or sagging of the skin, which all indicate aged skin. Thus, much of cosmetic science is targeted at treating underlying conditions that cause or stimulate the signs of skin aging.
The underlying conditions causing signs of skin aging can be characterized as intrinsic and extrinsic. Intrinsic causes of skin aging are inevitable and genetically determined; with age, the skin naturally becomes thinner and drier and important skin proteins, including collagen, responsible for maintaining the firmness of skin, and elastin, responsible for maintaining skin's elasticity, are degraded, reduced, and not replaced as quickly as in young skin. Extrinsic causes of skin aging include exposure to sunlight, ultraviolet (UV) light, lifestyle choices such as cigarette smoking, diet, exercise, stress levels, and amount of sleep. Evidence suggests that both intrinsic and extrinsic causes of aging lead to cumulative oxidative damage, which is incurred throughout one's lifetime and ages the skin. For example, UV light generates reactive oxygen species (ROS) in the skin. Oxygen free radicals or ROS are highly reactive species which are known to be a major factor in cell injury via oxidation and subsequent function impairment of lipids, proteins, and nucleic acids. ROS include hydrogen peroxide (H2O2), the superoxide anion (O2−), and free radicals such as the hydroxyl radical (OH−). These molecules are unstable and highly reactive, and can damage cells by chemical chain reactions such as lipid peroxidation. Indeed, active oxygen has been suggested as a major cause in not only aging but also several diseases including heart disease and cancer.
Free radicals, ROS, and RNS are known to degrade elastin and collagen in the skin and to decrease the ability of fibroblasts to produce collagen. Collagen is the primary protein of skin. Collagen (in a pre-processed form called pro-collagen) is assembled in cells and consists of three polypeptides wound around each other in a triple helix form, which is stabilized by intrachain disulfide bonds. After the helical molecule is assembled and modified in the cell it is secreted into the extracellular medium and further processed to a mature form (tropocollagen). Matured collagen molecules assemble into fibrils in the extracellular space in a staggered, parallel, fashion wherein the molecules are stabilized in this fibril pattern by covalent cross-linking bonds between the N-terminus of one molecule and the C-terminus of another. The collagen fibrils are interlaced and branched in skin. These interlaced, branched collagen fibrils provide the skin with its shape and firmness. Thus, when collagen is degraded or lost, skin looses its firmness and becomes lax. Another skin protein, elastin, coils and recoils like a spring and accounts for the elasticity of skin. Elastin is normally not produced by the human body after puberty and aging begin. Therefore, to reduce the visible signs of skin aging, such as loss of resiliency, which causes sagging of the skin, it is important to prevent degradation or loss of elastin and/or to stimulate production of elastin.
Like loss of collagen and elastin due to oxidative damage from free radicals, ROS, and RNS, loss of moisture and increased inflammatory responses contribute to skin aging. Indeed, the skin's capacity to inhibit inflammatory responses and retain water decreases with age, making the skin more vulnerable to dehydration and wrinkling. Lipids and fats in the skin help combat water loss by providing an epidermal barrier. This barrier hinders the growth of bacteria, which can cause skin irritation and sensitivity, which leads to increased inflammation and contributes to aging of skin.
The skin employs a host of protective mechanisms to defend itself against the ravages of the environment, free radicals, ROS, and RNS. One of the most widely studied protective mechanisms is the system of free radical scavengers. Free radical scavengers help to protect the skin by neutralizing dangerous substances, such as ROS and RNS, which are generated by sun exposure and pollution. Therefore, a composition containing free radical scavengers along with compounds that stimulate or increase synthesis or production of important skin proteins, such as collagen and elastin, or compounds that reduce or prevent loss or degradation of such proteins are useful for hydrating the skin, inhibiting inflammation of the skin, and lightening or increasing the evenness of skin tone and thereby for improving the appearance, texture, and moisture of the skin and for maintaining general skin health.